Aircraft gas turbine engines including auxiliary power units (APUs) and main propulsion engines may incorporate dual alloy turbine (DAT) rotors. A conventional dual alloy turbine rotor comprises a blade ring made of a first alloy having a desired characteristic and a hub of a second alloy having another different desired characteristic. For example, hubs have been formed from alloys that have high tensile strength and low-cycle fatigue resistance. Blade rings that are exposed to the higher temperatures of the combustion gas path and higher centrifugal loads have been integrally cast as one piece (hereinafter a “unitary blade ring”) from equi-axed alloys that have high stress rupture and creep resistance. The hub is fabricated separately from the blade ring. Hot isostatic pressing (HIP) facilitates diffusion bonding of the two dissimilar alloy components (the blade ring and the hub) to form the dual alloy turbine rotor. Vacuum sealing the interface between the blade ring and the hub is necessary for acceptable diffusion bond formation. Metal containers or “cans” have been used to completely enclose and vacuum seal the interface and dual alloy components during HIP. Unfortunately, these containers are unsuitable for some applications due to container leakage and geometric limitations.
In another known method (the so-called “shrink fit method”) for manufacturing a dual alloy turbine rotor, the cast unitary blade ring and hub are assembled with an interference fit. The interface between the cast unitary blade ring and hub is vacuum sealed by brazing to form a braze joint. Braze material is deposited in a shallow groove at the external surfaces of the interface between the cast unitary blade ring and hub. A subsequent vacuum braze thermal cycle completes braze joint formation. The assembled cast unitary blade ring and hub are then bonded together by the HIP process. The interference fit between the cast unitary blade ring and the hub is used to protect the integrity of the fragile braze joint, i.e., the interference fit is used to ensure that no gapping of the interface occurs during the vacuum braze thermal cycle and subsequent cool down. The interference fit imposes hoop stresses on the cast unitary blade ring when the cast unitary blade ring and hub are at the same temperature. The hoop stresses can potentially get worse depending on the relative coefficients of thermal expansion and temperature distribution during the vacuum braze thermal cycle.
In addition, in some gas turbine engines, it would be desirable to use single crystal or directionally solidified (DS) blade airfoils in the blade ring, rather than equi-axed alloy blade airfoils. Single crystal superalloys offer superior high temperature creep strength, for example. However, it is technically difficult to cast unitary blade rings with single crystal and directionally solidified alloys. In addition, while individual single crystal and directionally solidified airfoil blades may be brazed or diffusion bonded into an assembled blade ring (in contrast to a unitary blade ring), the blade-to-blade joints (hereinafter “blade braze joints”) in the assembled blade ring lack sufficient strength to withstand the high tensile hoop stresses imposed in the shrink-fit method for manufacturing the dual alloy turbine rotor. More specifically, with the interference fit and a higher thermal expansion of the hub relative to the assembled blade ring during a vacuum braze thermal cycle, the assembled blade ring is subject to fracture because of the high tensile hoop stresses. Therefore, manufacture of a dual alloy turbine rotor including an assembled blade ring of a single crystal alloy, a directionally solidified alloy, or an equi-axed alloy has not been possible.
Hence, there is a need for dual alloy turbine rotors and methods for manufacturing the same. There is also a need to minimize stress on the assembled blade ring during vacuum sealing of the interface between the assembled blade ring and hub, thereby substantially preventing assembled blade ring fracture and allowing the use of single crystal, directionally solidified, or equi-axed alloy blade airfoils in an assembled blade ring of the dual alloy turbine rotor.